The present invention generally relates to an electrical trap and more particularly, to a so-called "asymmetrical trap" to be used, for example, in an ultra-high frequency (UHF) section of an electronic circuit to reject undesired frequencies.
Conventionally, there has been available an electrical trap adapted to attenuate a signal in a region of a specific frequency as shown by a solid line curve T in FIG. 1(aa) by producing a series resonance at such a specific frequency through the connection of a resonant line between a hot line (e.g. a line through which a signal is transmitted) and a cold line (e.g. a line which is grounded) of a signal transmission line. An electrical trap in which modes of attenuation at opposite side regions of a trap frequency f.sub.t are similar to each other as shown by the solid line curve T in FIG. 1(a), is generally called a symmetrical trap. Meanwhile, as shown by a solid line curve T' in FIG. 2(a), an electrical trap in which modes of attenuation at opposite side regions of the trap frequency f.sub.t are different from each other due to presence of an anti-resonant frequency f.sub.p at a frequency region higher or lower than the trap frequency f.sub.t, is sometimes referred to as an asymmetrical trap.
The anti-resonant frequency f.sub.p as described above may be utilized in the case where there is a frequency region which is not desired to be subjected to attenuation in the vicinity of the trap frequency f.sub.t.
More specifically, for example, as shown in the frequency-attenuation diagram of FIG. 1(a), a dotted line characteristic curve F contains a spurious component as shown at Fs, and in such a case, the trap is combined with a band-pass filter in order to suppress such an undesirable spurious component. In the above case, the characteristics represented by the solid line curve T at a frequency region higher than the trap frequency f.sub.t, vary in a gentle curve, thus adversely affecting the pass-band of the band-pass filter, and consequently, a dotted line characteristic curve F' of the band-pass filter is undesirably deviated from the center frequency f.sub.c as shown in FIG. 1(b). In order to eliminate the disadvantage as described above, it has been proposed to employ the asymmetrical trap in which the characteristics of the solid line curve T at the region higher than the trap frequency f.sub.t vary sharply as shown by a solid line curve T' in FIG. 2(a). By the employment of the asymmetrical trap as described above, the undesirable spurious component may be advantageously suppressed without deviation of the characteristic curve of the band-pass filter as shown in FIG. 2(bb).
Incidentally, in the conventional asymmetrical traps, the anti-resonant point f.sub.p as referred to above may be realized by connecting a reactance component in parallel with respect to the resonant line, and is displaced as shown by arrows m in FIG. 2(a) depending on values of the reactance component employed. In an electrical filter employing a re-entrant cavity resonator, it has been a common practice to set the anti-resonant point through alterations of a coupling method by changing configurations and positon of a probe at the coupling point thereof. However, electrical traps employing dielectric material coaxial resonators and capable of controlling the anti-resonant points thereof have not yet been put into actual application up to the present.